(1) Field of the Invention
The present invention relates generally to gas sensors, and more particularly to ionic liquid piezoelectric gas sensors. Specifically, the present invention relates to ionic liquid piezoelectric gas sensors which are capable of detecting both polar and nonpolar organic vapors at high temperature and which have a fast linear and reversible response.
(2) Description of the Related Art
Identifying and correcting emissions from high-polluting vehicles requires small sensors working at high temperatures to monitor pollutants in exhaust gas or leaking fuels (Tsang et al., J. Phys. Chem. B, 2001, 105, 5737-5742; Kaltenpoth et al., Anal. Chem., 2003, 75, 4756-4765). High temperature gas sensing is conventionally achieved by using semi-conductive metal oxides, such as SnO2 and TiO2 (Dutta et al., J. Phys. Chem. B, 1999, 103, 4412-4422; Ikohura and Watson, The Stannic Oxide Gas Sensor; CRC Press: Boca Raton, Fla., 1994; Zhu et al., Anal. Chem., 2002, 74, 120-124). The resistance of metal oxides changes in the presence of organic vapors, CO or H2. It takes relatively a long time to reach equilibrium for the sorption of analytes from gas phase onto the metal oxides, especially for porous materials. The dependency of the resistance of the metal oxides on the vapor concentration is not linear, which reduces the accuracy of quantitative analysis (Simon et al. J. Comb. Chem., 2002, 4, 511-515). Some metal oxides work only at temperatures higher than a “switch on” value, e.g. >700° C. for SrTiO3 (Hu et al., J. Phys. Chem. B; 2004, 108, 11214-11218; Wang et al., J. Am. Chem. Soc., 2003, 125, 16176-16177; Dutta et al., Chem. Mater., 2004, 16, 5198-5204).
Rubbery polymers with low glass transition temperatures (Tg) have been used as coatings for detection of nonpolar or weakly polar organic vapors (Grate et al., Anal. Chem., 1993, 65, 987A). The vapor sorption in rubbery polymers is reversible and equilibrium is attained rapidly (Grate et al., Anal. Chem., 1993, 65, 987A; (a) Jarrett and Finklea, Anal. Chem., 1999, 71, 353; (b) Shinar et al., Anal. Chem., 2000, 72, 5981; (c) Zellers et al., Anal. Chem., 1995, 67, 1092; (d) Patrash and Zellers, Anal. Chem., 1993, 65, 2055). However, the mechanical properties of rubbery polymers strongly depend upon temperature (U. W. Gedde, Polymer Physics, Kluwer Academic Publ., Doedrecht, Netherlands, 1999). Most polymer materials with low Tg are not stable at high temperatures. Therefore, applications of polymer materials for high temperature vapor sensing are limited. Furthermore, if the vapors cannot absorb on the materials, the large surface-area to volume ratio sensing materials, such as graphite ((a) Jarrett and Finklea, Anal. Chem., 1999, 71, 353; (b) Shinar et al., Anal. Chem., 2000, 72, 5981; (c) Zellers et al., Anal. Chem., 1995, 67, 1092; (d) Patrash and Zellers, Anal. Chem., 1993, 65, 2055) or oxides (Dutta et al., J. Phys. Chem. B, 1999, 103, 4412-4422; Ikohura and Watson, The Stannic Oxide Gas Sensor; CRC Press: Boca Raton, Fla., 1994; Zhu et al., Anal. Chem., 2002, 74, 120-124) would not work for high temperature gas sensing.
U.S. Pat. No. 4,236,893 to Rice, U.S. Pat. No. 4,242,096 to Oliveira et al., U.S. Pat. No. 4,246,344 to Silver III, U.S. Pat. No. 4,314,821 to Rice, U.S. Pat. No. 4,735,906 to Bastiaans, and U.S. Pat. No. 6,087,187 to Wiegland et al. each teach using a piezoelectric sensor for the detection of an analyte in a liquid sample. U.S. Patent Application Publication Nos. 2003/0077222, 2003/0073133, 2003/0072710, 2003/0068273, 2003/0053950, and 2003/0049204, all to Leyland-Jones, discloses immunosensors which in particular embodiments have antibodies, Fab fragments, or scFv polypeptides immobilized on the surface thereof.
U.S. Patent Application Nos. 2002/0094531 to Zenhausern teach sensing probes such as a QCM for detecting a biological analyte of interest in gaseous, vapor, or liquid forms. The sensing probes are coated with various materials, such as polymers, ion exchange resins, porous silicon, silanes, thiols, and oxides. However ionic liquids are not taught as a coating for the sensing probes.
U.S. Patent Application Nos. 2002/0142477 to Lewis et al. teach organic vapor measurement using a polymer-coated quartz crystal microbalance. The quartz crystal microbalance crystals are coated with polymers including poly (ethylene-co-vinyl acetate) with 25% acetate (PEVA) and poly(caprolactone) (PCL) polymer films.
Liang et al. (Anal. Chem., 2002, 74, pp. 2172-2176) teach a quartz crystal microbalance device which employs an ionic liquid for sensing organic vapors. Liang et al. used an ionic liquid having 1-R1-2-R2-3-methylimidazolium cations where R1 is ethyl, propyl, or butyl and R2 is H or methyl in a system which operated at a constant temperature of 25° C. The QCM sensors coated with the ionic liquids had a linear response only in the narrow concentration range from 2.3×104 to 3.8×105 ppm. The linearity of the frequency shift versus the concentration did not extend to low concentrations or high concentrations.
While the related art teach gas sensors and piezoelectric sensors for detecting analytes in liquids, there still exists a need for a superior gas sensor which are capable of detecting both polar and nonpolar organic vapors at high temperature which have a fast linear and reversible response.